Various receivers receiving signals in a first frequency band may operate in close proximity to other components, which operate at another frequency band but cause interfering higher harmonics and/or inter-modulation products within the first frequency band.
For example, there will be an increasing number of mobile devices, which comprise various combinations of cellular communication transceivers (combined transmitter and receiver) on the one hand and Orthogonal Frequency Division Multiplex (OFDM) receivers on the other hand, and in which higher harmonics of the cellular communication transceivers transmissions may fall into an OFDM reception frequency band.
OFDM is a modulation technique in which the original data is divided into several smaller data streams, each using a different narrowband sub-carrier in the frequency domain. Each sub-carrier is modulated with Binary Phase Shift Keying (BPSK), a Quadrature Phase Shift Keying (QPSK), a 16 Quadrature Amplitude Modulation (16QAM) or a 64QAM modulation scheme, as an example. For transmission, the signal is transformed into the time-domain. The OFDM receiver transforms the signal back into the frequency-domain for regaining the original data from the sub-carriers.
OFDM will be the dominant modulation technique in upcoming wireless standards. OFDM has already been selected as modulation technique for Wireless Local Access Networks (WLAN), Worldwide Interoperability for Microwave Access (Wimax), Ultra WideBand (UWB) and Digital Video Broadcasting Handhelds (DVB-H).
Currently, for example, there are WLAN and Global System for Mobile communications (GSM) and Bluetooth™ transceivers integrated into the same mobile terminal.
In future handheld devices, a DVB-H receiver may be integrated for instance along with a cellular mobile phone operating according to the GSM standard or to the Wideband Code-Division Multiple Access (WCDMA) standard.
An exemplary system 100 employing a GSM transceiver 110 and an OFDM receiver 150 is illustrated in FIG. 1.
The GSM transceiver 110 comprises an antenna 111, which is connected to a component 112 having the functionality of a duplexer. The functionality of duplexer component 112 may be implemented by radio frequency switches. The component 112 connects on the one hand the antenna 111 to a reception chain and on the other hand a transmission chain to the antenna 111.
In the reception chain, the duplexer 112 is connected via a variable gain low noise amplifier 121, a downconversion mixer 122, a baseband filter 123 and an analog-to-digital (A/D) converter 124 to a digital signal processor (DSP) 114.
The output of the A/D converter 124 is linked within the DSP 114 via a bit detection block 126 to a decoding block 127. The DSP 114 further comprises a received signal strength indication (RSSI) determination block 128. The outputs of the A/D converter 124 and of the bit detection block 126 can be linked in addition to the input of the RSSI determination block 128, which controls the variable gain low noise amplifier 121.
In the transmission chain, a coding block 137 of the DSP 114 is connected via a bit generation block 136 of the DSP 114 to a digital-to-analog (D/A) converter 134. The D/A converter 134 is connected via a baseband filter 133, an upconversion mixer 132 and a variable gain power amplifier 131 to the duplexer 112. A local oscillator 113 is connected to both downconversion mixer 122 and upconversion mixer 132.
The OFDM receiver 150 comprises an antenna 151, which is connected via a variable gain low noise amplifier 161, a downconversion mixer 162, a baseband filter 163 and an A/D converter 164 to a DSP 154. The output of the A/D converter 164 is linked within the DSP 154 via a Fast Fourier Transformation (FFT) block 165 and a bit detection block 166 to a decoding block 167. The DSP 154 further comprises an RSSI determination block 168. The outputs of the A/D converter 164 and of the FFT block 165 can be linked in addition to the input of the RSSI determination block 168, which controls the variable gain low noise amplifier 161.
A problem that may occur with such a combination is that harmonics of transmissions by the GSM transceiver 110 occur within the radio frequency (RF) band supported by the OFDM receiver 150. The interference level which the GSM transceiver 110 generates may be dozens of decibels higher than the received OFDM signal level.
The currently used WLAN standard 802.11b and Bluetooth™ use the 2.4 GHz frequency band, which suffers from the third harmonic of GSM850 transmissions at the highest US channel.
When the WLAN standard 802.11a using the 5 GHz band is integrated into mobile terminals, the problems will expand. In this case, the third harmonic of GSM1800 transmissions will hit to the WLAN reception/transmission band.
While in WLAN there are eleven channels, in the US DVB-H system there will be only one RF channel at 1670-1675 MHz. The most severe problem will thus occur with DVB-H in the U.S.A., since the second harmonic of the GSM850 band transmissions will hit to the single DVB-H US channel, while the use of another channel is not enabled.
In the upcoming Wimax standard in Europe, most probably the 3.5 GHz band will be deployed, which will suffer from interferences due to the second harmonic of GSM1800 transmissions.
The interference problem is illustrated for two scenarios in FIG. 2. FIG. 2 is a diagram illustrating various power levels in dBm for DVB-H, WLAN, GSM and WCDMA over frequency.
GSM850 operates at frequencies around 850 MHz with a transmission power level of +33 dBm. GSM1800 operates at frequencies around 1800 MHz with a transmission power level of +30 dBm. WCDMA2100 operates at frequencies around 2100 MHz with a transmission power level of +21 dBm. The mobile terminal transmission power levels may be varied dependent on the supported power class of the product.
US DVB-H operates in a frequency range of 1670-1675 MHz. As indicated in FIG. 2, the second harmonic of GSM850 transmissions falls into the US DVB-H channel with a maximum power level of −30 dBm, depending on an attenuation between transmission and reception antennas. This is in line with the GSM/WCDMA transmission spurious emission specification, which sets a limit of −30 dBm. The minimum signal level for DVB-H, however, is −95 dBm. Thus, there may result a negative signal-to-interference (S/I) ratio for a US DVB-H reception in case of an ongoing GSM850 transmission in close vicinity, for example in the same device.
WLAN 802.11a operates in a frequency range of 5130-5725 MHz in Europe, of 5180-5825 MHz in US and of 4190-5350 MHz in Japan. As indicated in FIG. 2, the third harmonic of GSM1800 transmissions falls into the WLAN 802.11a band with a power level of about −60 dBm. The minimum signal level for WLAN, however, is less than −90 dBm. Thus, there may result a negative S/I ratio for the WLAN reception in case of an ongoing GSM1800 transmission in close vicinity.
The occurrence of further harmonics, which may result in interferences in other frequency bands, is only indicated for completeness.
Similar problems may occur when a UWB receiver is integrated into the same device as the transmitter or transceiver of some other system. A media terminal may enable for example a voice and data connections using GSM or WCDMA and in addition data connections using UWB.
When the UWB system was specified, it was a target that UWB does not interfere with other systems. It was not considered, in contrast, how other systems will interfere with UWB receptions in combined terminals.
Possible interference scenarios occurring in case a UWB receiver and a GSM/WCDMA transceiver are integrated into the same device are illustrated in FIG. 3. FIG. 3 is a diagram indicating various power levels in dBm for GSM, WCDMA and UWB over frequency.
In a UWB receiver, a reception band filter following upon the antenna filters out frequencies which are below 3.1 GHz and above 10.6 GHz. Thus, the fundamental frequencies of GSM900 transmissions having a power level of +33 dBm, of GSM1800 transmissions having a power level of +30 dBm and of WCDMA2000 transmissions having a power level of +21 dBm are suppressed.
However, the power level of generated second to fifth harmonics of GSM1800 and WCDMA2000 transmissions is much higher than the power level of a received UWB signal.
This high interfering power level corrupts the UWB signal. As a result, the UWB data throughput is lowered or completely stopped, depending on received UWB signal strength.
Also the fifth and higher harmonics of GSM900 transmissions, not shown in FIG. 3, will fall into the UWB band.
Interference situations that may occur in case a UWB receiver and a WLAN transceiver are integrated into the same device are illustrated in FIG. 4. FIG. 4 is a diagram indicating various power levels for UWB and WLAN over frequency.
Due to the above mentioned filtering in a UWB receiver, the fundamental frequencies of WLAN 802.11b transmissions having a power level of +20 dBm can also be suppressed. Still, the suppression level is lower than in the case of GSM and WCDMA, since there is only a transition band of about 600 MHz from the highest WLAN frequency to the lowest UWB frequency. Further, the second harmonic of WLAN transmissions at a power level of about −40 dBm, the third harmonic of WLAN transmissions at a power level of about −60 dBm and the fourth harmonic of WLAN transmissions cause interferences in the UWB band.
WLAN 802.11a operating at 5 GHz uses a frequency band within the UWB frequency band with a power level of +20 dBm. Consequently, those two systems can not operate at the same time, since transmitted WLAN signal will saturate the low noise amplifier of the UWB receiver completely.
It might be noted that a second, alternative UWB standard proposal by the multiband OFDM consortium is based on OFDM. This is illustrated in FIG. 5. FIG. 5 is a diagram illustrating thirteen radio channels of 528 MHz each. The channels are divided into four groups A-D. In the proposed UWB specification, each 528 MHz radio channel is divided into 128 sub-carriers.
Known approaches for dealing with interferences typically consider only situations in which the desired signal has a higher level than the interference so that the S/I ratio is positive.
In PCT application WO 03/105386, it has been proposed to avoid interference by switching off a receiver of a device while a transmitter of the device is sending out a transmission burst.
In PCT application WO 04/109942, it has been proposed to minimize a wide band noise interference from the first transmitter to the second receiver by changing antenna tuning of the second receiver from the first frequency band to second frequency band when wide band noise is expected at the first frequency band.
In US patent application 2003/0235254, it has been proposed to identify a channel that is jammed by a narrowband jamming signal by comparing the signal in each channel to an average of powers of adjacent channels. Channels that are identified as being jammed are included in a jam mask for use in subsequent processing.